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Rainfall – runoff modelling in the urban catchment of El Batan, Quito, Ecuador

Perrin, JL / Bouvier, C

The El Batan urban watershed covers about 50?km² in Quito, Ecuador. Rainfall and stream flow data were collected between 1982 and 1984 and then again in 1998. Although both geometric and hydraulic data were available, rainfall?–?runoff modelling was carried out using a conceptual rainfall?–?runoff model in order to avoid the very tedious work that defining the topology (pipes, channels, nodes, subcatchments) of such a large area implies. Considering both the physiographic heterogeneity and the high variability of the rainfall in this mountainous context, a distributed modelling approach was chosen that develops and generalizes the concept of the unit hydrogram to a grid-cell structure. For each cell, the contribution at a given time is computed by using an instant runoff coefficient. Then the elementary contributions are transferred through the drainage system by means of a translation and storage combination, which uses two parameters. The runoff coefficient primarily depends on the land-use characteristics, so this coefficient changes appreciably between 1982?–?1984 and 1998 due to urban growth and the development of drainage infrastructure. Discharge parameters are stable for the whole set of events and for both observation periods, and can be related to hydraulic characteristics of the drainage network. On the basis of simplified assumptions, it is shown that the distributed model is able to simulate the observed floods. Finally, the model was applied to the systematic detection of overloading in the Quito sewage network.